How do oceans act as carbon sinks for atmospheric carbon dioxide?

Oceans serve as vital carbon sinks by absorbing approximately 30 percent of annual anthropogenic carbon dioxide emissions. They function through two primary mechanisms:12

• Biological Pump: Phytoplankton at the ocean surface perform photosynthesis, fixing carbon into their tissues. When these organisms die, they sink to the deep ocean, effectively sequestering the carbon away from the atmosphere for long periods. While some carbon is eventually returned to the surface through upwelling, much of it remains stored in the deep sea or becomes sedimentary rock.13
• Solubility Pump: This process involves the physical transfer of dissolved CO₂ from shallow surface waters to deeper ocean layers, often driven by global ocean circulation patterns.4

However, the efficiency of these sinks is threatened by global warming. Rising temperatures reduce the solubility of CO₂ in surface waters and may slow oceanic circulation, while increased ocean acidity harms organisms that rely on calcium carbonate, potentially weakening the biological pump.4